Emulsified oil/water type metal working fluids are well known in the art and generally comprise a suspension of oil droplets in water containing emulsifying agents and other materials such as corrosion inhibitors and/or pH stabilizing agents. For purposes of this invention, the term "oil/water emulsion" includes both oil-in-water emulsion where the oil is commonly added to the water with the water being the continuous phase and the oil the discontinuous phase (i.e. droplets) and water-in-oil emulsion in which the phases are reversed and are commonly called invert emulsions. The oil constituant may be petroleum based paraffinic and/or naphthenic oil and often constitutes at least about 60% by volume of the metal working fluid. The term "oil" may also apply to petroleum like fluids such as polyolesters, polyalpha olefins, trigycerides and the like, where as these fluids are known for their lubricity qualities and are generally not water miscible. These fluids are typically referred to as synthetic lubricants. Emulsified oil generally has a milky appearance and contains oil particles having diameters larger than about 3 microns and commonly from about 80 microns to about 200 microns.
Numerous emulsifying agents are known in the art of which one example is an alkyl benzene sulphonate such as polyisobutene succinimide disclosed in U.S. Pat. No. 4,778,614, the disclosure of which is incorporated herein by reference. The referenced patent also discloses examples of other materials commonly added to oil/water emulsions such as defoaming agents like a suitable Freidel Krafts wax. Suitable emulsion defoaming agents based upon silicon may for example be purchased from the Dow Chemical Company. Emulsifying agents are more broadly disclosed in U.S. Pat. No. 4,822,507, the disclosure of which is incorporated herein by reference, where they are described as anionic surface active agents such as fatty acid soaps, sulfonates and naphthenic acid soaps or as cationic surface active agents such as long chain primary amine salts and alkyltrimethyl ammonium salts or as nonionic surface active agents such as polyoxyethylene alkyl esters and polyethylene esters and as phosphorous containing agents such as dipolyoxyethylene alkyl ether phosphate or dipolyoxyethylene alkylphenyl ether phosphate.
Commonly, about 15% by weight of one or more emulsifying or coupling agents is added to the water along with lesser amounts of other additives such as defoaming agents, biocides, pH stabilizers such as calcium carbonate and primary, secondary and tertiary amines, and corrosion inhibitors such as mixed alkanolamine borate described in U.S. Pat. No. 4,778,614, the disclosure of which is incorporated herein by reference.
In some instances the oil/water emulsion may further include animal or vegetable oil or synthetic fluids as previously described and/or extreme pressure additives such as sulfurized fats and oils such as sulfurized lard, sulfurized sperm oil, sulfurized caster oil and phosphates such as tributyl phosphate in amounts ranging from about 5 to above 30% by weight of the emulsion composition disclosed in U.S. Pat. No. 4,822,507 referenced earlier. Oil/water compositions are characteristically emulsified in high speed mixers such as homogeneous blenders and possess the combined advantage of lubricity and corrosion inhibiting ability provided by the oil droplets and cooling ability provided by the water component making them applicable to a variety of metal grinding and cutting applications over broad speed ranges.
After a period of use however, the oil/water emulsions become contaminated with machine oil, commonly called tramp oil as well as with metal particles, metal cations, anionic salts and other foreign matter rendering them progressively unsuitable for continued use in machining operations.
Generally, oils are able to remain in a stable emulsion dispersion because of the presence of negatively charged emulsifying agents such as soaps and petroleum or synthetic sulfonates previously described. The negatively charged emulsifying agents, in theory, are attacked by the positively charged cations present and operate to ultimately neutralize the charges on the emulsifiers present in the metal working fluid.
Although anionic salts such as carbonates, sulfates, chlorides and phosphates may also be present in the metal working fluid, they are believed to operate to increase electrical conductivity and not to significantly interfere with the metal working fluid emulsion stability.
It has been the practice heretofore to either discard the contaminated fluid entirely or to separate the oil from the water component with filters such as microfilters such as where the water component can then be discarded at sea as disclosed in U.S. Pat. No. 4,846,976, the disclosure of which is incorporated by reference. Filters are also employed in U.S. Pat. Nos. 4,655,927 and 4,016,076, to separate oil from the contaminated emulsified metal working fluid and the disclosures of both of which are incorporated herein by reference. In U.S. Pat. No. 4,655,927, the filter is described as a cross-flow ultrafilter containing a polymeric (nylon) porous membrane that is of the type that can be used to advantage in the present invention.
Ultrafiltration of synthetic water based metal working fluids (not oil/water emulsions) and the maintenance of cross-flow ultrafilters is described in separate articles in Volume 47, 8 on pages 653-659 and on pages 686-690 of "Lubrication Engineering" (August, 1991).
However, in neither the aforedescribed Journal Articles nor patents is it taught or suggested that one could take reclaimed oil from a contaminated oil/water emulsion and again enhance the emulsion properties by adding a predetermined amount of water having an electrical volume resistivity of at least about 3000 ohm-cm.
Phase separation or layering of the oil and water components of the contaminated oil/water emulsion characteristically triggers concern about discarding the fluid under consideration. Heretofor neither high speed agitation nor high speed shearing nor the addition of additional emulsifying or coupling agents has been effective to suspend the oil droplets uniformly throughout the metal working fluid once the emulsifier charges have been neutralized as previously described. Nor has the re-introduction of the permeate effluents separated from the oil in an ultrafiltering operation been able to return the fluid to a uniform dispersion of oil droplets suspended in water.
In view of the environmental problems associated with discarding oil that is characteristically slow or resistant to degradation by micro-organisms, the present invention is of great value in the reclaiming of such oil emulsion for re-use as an emulsified oil/water metal working fluid derived by the addition of water thereto having a electrical volume resistivity of at least about 3000 ohm-cm.
Although not completely understood, it is believed that phase separation or layering of the contaminated metal working fluid is caused by the presence of increasing amounts of cations and anions present in the fluid as previously described and that such, along with water, are able to permeate through the filter membrane leaving behind the oil with substantially reduced amounts of such cations and anions rendering the oil again capable of providing a useful reconditioned oil/water emulsion type metal working fluid. Heretofor it was thought by those in the oil/water emulsion metal working industry that cations became attached such by ionic bonding to the outer surface of the individual oil droplet and would thus remain with the oil during a filtration process rather than pass through the filter and continue to render the oil unsuitable for use in making further emulsions since the presence of the cations led to deliterious layering of the oil and water. Surprisingly, it has been discovered that a substantial amount of the cations and anions does in fact separate from the oil in an ultrafiltering process hereinafter described and thus enables further use of the oil for forming another oil/water emulsion by replacing the contaminated water phase with uncontaminated water.
It is also believed that contamination of the oil/water emulsion with tramp oils containing zinc dialkyldithiophosphate (ZDP) releases both zinc and phosphate into the emulsion that respectively lend and leave themselves significantly to emulsion instability.